A toothed power transmission member, such a member suitably being used in a gear, a spline, a screw, or a ball screw, is a machine part adapted to transmit a power to a toothed counter part power transmission member. In the toothed power transmission member, large surface pressure causes not only the contact between tooth surfaces but also a relative sliding therebetween. Accordingly, it is necessary for preventing the wear of the tooth surfaces and the power loss therebetween and for achieving a smooth power transmission therebetween that a lubricating oil (or a lubricating agents is always present on a tooth surface (or an engaging surface) portion.
Several methods are known as a method for supplying a lubricant to a tooth surface. One of the methods is to form a single or a plurality of oil reservoirs on a tooth surface, and to naturally diffuse the lubricant on the tooth surface from the oil reservoir. Examples are shown in Patent Document 1 to Patent Document 3. Such lubricating method is traditionally contrived and has advantages in that the principle thereof is simple, and that the interval of the maintenance, such as the supply of lubricant, can be lengthened. However, a low-cost processing method which can be used in mass-production is not realized since the processing of a groove on a tooth surface having a complex curved surface is so difficult.
In a case where oil reservoirs are formed in toothed power transmission members, such as pinions, racks, and screw shafts, oil reservoirs are formed on surfaces of, for example, involute teeth or tooth threads of such machine elements serving as transfer mechanisms. The processing of a groove in a surface of a tooth can be achieved by performing, after processing the tooth by means of a tooth cutting tool such as a hob, cutting, laser-processing, and electro-discharge processing. However, as the number of grooves increases, the processing of the grooves in addition to the creation of a tooth becomes more difficult, so that the processing thereof cannot efficiently be achieved.
Next, a steering apparatus is described below as an example of an apparatus that needs a toothed power transmission member.
In a steering apparatus, a telescopic shaft, which includes a male spline shaft and a female spline shaft that are fitted so as to be unable to rotate and as to be relatively slidable, is incorporated into an intermediate shaft or a steering shaft. Such telescopic shafts are disclosed in Patent Document B, Patent Document 9, Patent Document 10, and Patent Document 11.
FIGS. 33 to 35 illustrate a conventional telescopic shaft. FIG. 33 is a front view illustrating a conventional steering apparatus having a telescopic shaft. FIG. 34 illustrates a male spline shaft constituting an intermediate shaft shown in FIG. 33, wherein (1) illustrates a front view of the male spline shaft, and (2) is a cross-sectional view taken along line A-A shown in (1). FIG. 35 illustrates a female spline shaft constituting the intermediate shaft shown in FIG. 33, wherein (1) is a longitudinal cross-sectional view of the female spline shaft, and (2) is a right side view of the female spline shaft shown in (1).
An intermediate shaft 12 and a steering shaft 13, which serve as telescopic shafts, are incorporated into a steering apparatus shown in FIG. 33. When a cardan joint 17 is fastened to a pinion shaft 16 engaging with a rack shaft 15 of a steering gear 14, the intermediate shaft 12 is temporarily contracted and is then fitted to the pinion shaft 16, thereby being fastened thereto. Thus, the intermediate shaft 12 needs a telescopic function. Also, the intermediate shaft 12 is required to have capabilities of absorbing the axial displacement and vibrations occurring while a vehicle is running, and of preventing the displacement and vibrations from being transmitted to a steering wheel 18. Because of necessity to shift the position of the steering wheel 18 according to the physique and the driving posture of a driver, the steering shaft 13 is required to have a function of axially expanding and contracting.
FIG. 34 shows a male spline shaft 121 constituting the intermediate shaft 12. FIG. 35 shows a female spline shaft 122 constituting the intermediate shaft 12. The telescopic shaft constituted by the male spline shaft 121 and the female spline shaft 122 needs to reduce a sliding resistance, which is caused while expanding and contracting, to thereby smoothly be expanded and contracted with a small force. Therefore, as described in Patent Document 8 and FIG. 34(2), the periphery of the tooth surface of the male spline shaft 121 is coated with a resin 123, whose sliding resistance is small. Then, a lubricating agent is applied thereto. Subsequently, the male spline shaft 121 is fitted thereinto to thereby assemble the telescopic shaft,
Such a telescopic shaft is iteratively expanded and contracted by simultaneously applying torque thereto while driving the vehicle. Thus, it is necessary to constantly supply a lubricating agent (or lubricating oil) to a contact portion between the tooth surfaces of the male spline and the female spline. As a resolution therefor, the tooth surface of the male spline shaft is coated with resins to thereby form a two layer structure having upper and lower layers, as described in Patent Document 10. Also, a space communicating with the outside is formed in the upper resin layer so that a lubricating agent is stored in this space. However, this conventional member has a drawback in that because a manufacturing process is complex, the manufacturing cost thereof rises.
Also, as described in Patent Document 11, there is a resin-coated sliding spline constructed so that a concave portion is dug to extend in a circumferential direction of the tooth surface of the female spline, and that an oil reservoir corresponding to this concave portion is formed by adjusting the thickness of a resin coating film to be coated on this concave portion. However, this technique is to form a concave portion in the tooth surface of the female spline after the female spline is formed. Thus, it is difficult to form a concave portion having a constant depth along a very uneven tooth surface of the female spline. That is, in a case where the concave portion is processed by cutting, intermittent cutting is performed. Consequently, burrs are produced on the tooth surface of the spline. The elimination of the burrs is time-consuming. Also, the durability of a working tool is lowered. Thus, this technique has a drawback in that the manufacturing cost rises. Also, in a case where a concave portion is processed by plastic working, the shape of apart of the spline, which part surrounds the concave portion, is lost. Thus, this technique has another drawback in that the accuracy of dimension of the tooth surface of the spline is reduced.
Also, there is a method of enhancing the lubricating-agent holding capability according to the ability of the lubricating agent. However, the viscosity of the lubricating agent becomes high. This results in a high expansion/contraction resistance at assembling. Thus, this method has a problem in that the assembling is difficult to achieve.
Generally, a fluid lubrication is intended to reduce the friction of a sliding surface of a machine element, a meshing portion of a gear, and a screwing portion of a screw, to reduce the wear thereof by maintaining the thickness of an oil film, and to dissipate an amount of generated heat. In a case where a machine and an apparatus are operated by maintaining an appropriate thickness of an oil film, the wear of a sliding surface can be considerably reduced. The lifetime of a machine and an apparatus can be elongated. The thickness of an oil film always varies according to change of mechanical conditions, for example, a load and a speed, and to the influence of a used lubricating agent. Therefore, it is too early to see that lubrication requirements are satisfied by simply supplying a lubricating agent.
Especially, in the meshing portion of the gear of a power transmission mechanism, the surface pressure of the gear is high. Thus, change in the surface pressure of the gear has large influence on the formation of an oil film. As is well known, the higher the surface pressure of the gear rises, the smaller the thickness of the oil film becomes. Therefore, when a great load is applied on the tooth surface, a desirable fluid lubrication cannot be maintained. A lubricating mechanism changes from a fluid lubrication to a mixed lubrication by which larger friction occurs. At that time, the friction caused by the contact between a gear tooth and a counterpart gear tooth increases to thereby significantly increase the wear of the tooth surfaces.
It is preferable for suppressing the wear of the teeth of the gear that means for assisting the assurance of the thickness of an oil film is provided. There has been provided the technique of forming an oil reservoir, which holds lubricating oil, in a tooth of a gear of a worm wheel of a worm reducer (see, for example, JP-A-8-226526).
An oil film can immediately be recovered by supplying a lubricating agent, which is stored in the oil reservoir, to a tooth surface even when the tooth surface temporarily runs out of an oil film. Thus, an oil reservoir placed in a region, in which the tooth of a gear of a worm is brought into sliding contact with that of a gear of a worm wheel, can serve to stably form an oil film.
Patent Document 1: JP-A-2004-155223.
Patent Document 2: JP-A-2003-207031
Patent Document 3: JP-A-08-226536
Patent Document 4: JP-A-2002-079349
Patent Document 5: JP-A-11-010274
Patent Document 6: JP-A-07-310807
Patent Document 7: JP-A-07-308729
Patent Document 8: JP-A-2003-054421
Patent Document 9: JP-A-2004-324863
Patent Document 10: JP-A-2003-013986
Patent Document 11: JP-A-56-059030
Patent Document 12: JP-A-8-226526 (page 4, FIGS. 3-5)